Hematopoietic stem cells (HSCs) are a rare population of cells in blood forming tissues that can self-renew and differentiate into all blood cell lineages. The long-term objective of the candidate is to understand the underlying mechanisms and regulators of HSC fate decision and survival. We propose to study a pair of activating and inhibitory receptors named paired Ig-like receptors (PIRs) that are expressed in HSC. PIR-A (activating) and PIR-B (inhibitory) form a novel MHC class I recognition system that can constitutively regulate the activation threshold in cells that express them. PIR is involved in the regulation of cytokine, integrin, and chemokine receptor signaling pathways that are crucial for HSC and progenitor functions. Thus we hypothesize that PIRs are major regulators of hematopoietic stem cell self-renewal, migration and survival. Our preliminary findings have demonstrated that: 1) PIR expression is specifically regulated among hematopoietic HSC and progenitor cell subsets;2) Perturbing PIR expression, using a lentiviral shRNA knockdown strategy, dramatically reduces HSC re-engraftment capability;3) PIR-A knockdown HSCs display selective reduction in responsiveness to the CXCR4 ligand SDF-1a in in vitro chemotaxis assays; and 4) Knocking down PIR expression in mast cell and HSCs greatly alters their responses to SCF and IL-3, cytokines important for their survival. Based on these observations, we plan first to investigate whether altering expression levels of PIRs or PIR ligands will influence HSC engraftment capacity. Then we will try to unravel the underlying mechanisms of how PIR regulate HSC functions. Lastly, we will determine the role of PIR, as the only known MHC recognition system expressed on HSC,in HSC engraftment resistance between MHC-match and allogeneic mouse strains.